The invention relates to electrically heated panels. The invention is particularly applicable to electric blankets, and will be specifically described in relation thereto. However, it will be appreciated that the invention may also be applied to many other forms of heated panels, such as under-carpet heaters, mattresses and pads.
Electric blankets commonly include a heating element in the form of inner and outer concentric coil conductors separated by an electrically insulating layer. One or other, or both, of the coils provide the heating effect. Should overheating occur, the insulating layer separating the two conductors is designed to melt, causing the two conductors to come into electrical contact with one another, causing a short circuit or low impedance path to occur. This causes a surge of current in the electric circuit containing the conductors, causing a fuse to blow and thus isolating the blanket from the mains supply.
There are many ways of detecting that a short circuit or low impedance path has developed between the two conductors, and examples of suitable circuits are described in British Patent Specifications Nos. 1155118, 1585921, 1599709, and 2028608. However, the known arrangements suffer from certain disadvantages and there are three particular problem areas which the arrangements described in the above-mentioned specifications, and many other arrangements, fail to address.
Apart from incorporating an effective overheat protection system an electric blanket must, in order to be a practical commercial proposition, also incorporate means for providing selective multiple heat settings. The above-mentioned specifications therefore all show ways in which the two resistive conductors of the heating element can be switched or electrically connected so as to provide the required alternative heat settings.
In practice, in order to achieve the required heat settings the four, or more, ends of the conductors are connected to either an in-line switch or a bedside control unit. In some cases the coaxial heating conductors themselves are extended, within a protective sheath, from the blanket to the switch in which the required electrical connections are made. In other blankets the ends of the heating conductors are connected to conventional bunch stranded copper multicore cables which lead to the switch or control unit where they are connected to electric circuitry for controlling the multiple heat settings and the overheat protection circuitry. In some blankets an intermediate multi-pin plug and socket may be provided between the blanket and the connecting cable leading to the switch or control unit.
In both of these known arrangements the sheath type interconnection or multi-core cable is bulky and heavy and relatively stiff, particularly under cold conditions, and accordingly may be difficult to manipulate and stow away, leading to complaints from users as a result of the intrusive nature of the cables. More importantly, however, although electric blankets of this kind have led to a large reduction in fires caused by overheating of the blanket, there has been an increase in the number of fires caused by arcing due to failure of the electrical flex leading to the blanket. It is now well accepted that in the many blankets on the market today, which are well protected against overheating, the weakest part of the blanket is the multicore connection between the blanket and the switch or control box. This has been as a direct result of the provision of overheat protection because in early simple electric blankets which were used to preheat beds, and where overheat protection was not provided, it was possible to use simple slim two-core round flex, or flat twin cables, between the blanket and control switch or unit. The life of such cables, and resistance to fatigue and failure through flexing, is many times greater than that of, in particular, the bulky four core cables now conventionally required in blankets with overheat protection.
The incorporation of the above-mentioned advanced overheat protection systems in blankets has led to greater public confidence in using electric blankets, not just for pre-warming a bed, but also for all-night use as well. The increasing tendency for people to lie on or under electrically heated blankets has led to considerable research, mostly in U.S.A., on the question of whether being in close proximity to heating conductors, and the electro-magnetic fields which they radiate, has any adverse effect on the human body. Such research was initially triggered by the concern of many medical experts at the higher incidence of certain illnesses amongst people living in the vicinity of high voltage electric transmission lines. The field strengths under such power lines are very much higher than those emitted by electric blanket heating elements and to date no firm evidence has been found that these electro-magnetic fields can cause harm, although research is continuing. However, for the sake of public confidence, it is desirable that electric blankets should be so designed as to reduce to a minimum any electro-magnetic fields resulting from the electric current flowing through the heating elements.
Another disadvantage of known electric blankets having advanced overheat protection systems is the higher incidence of failure of an electrical component or fuse even though overheating has not occurred. The main reason for such failures is the transient breakdown of one of more of the components incorporated within the overheat protection circuitry.
Very often these overheat protection circuits incorporate diodes arranged "back-to-back" across the supply terminals. When a short circuit develops between the coaxial heating conductors, as a result of melting of the insulation layer, the increased current flow in these diodes causes a considerable increase in supply current, from a.c. to d.c., resulting in the blowing of a series fuse.
British Patent Specification No. 1585921 shows, for example, how the multi-heat configurations described in Specification No. 1155118 can be considerably improved, to provide reliable detection of a short circuit due to overheating at any position along the element length, by the fitting of a shunt diode in parallel with one or both of the heating conductors. In practice, however, it has been found that transient breakdown of any of the series diodes shown in FIGS. 2-4 of Specification No. 1585921 causes the series fuse to blow due to there being no limiting resistance in the fusing path. Thus, increasing the safety and reliability of the blanket with respect to overheating has led to increasing failures due to unforeseen control unit or switch component breakdowns caused by transient mains voltage perturbations.